The present invention relates to the art of gaseous separation. It finds particular application in conjunction with separating oxygen and nitrogen components of atmospheric air and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in conjunction with the separation of other components of gaseous mixtures.
Heretofore, pressure swing adsorption (PSA) gas concentrators have commonly included first and second molecular sieve beds connected by a cross over valve to an air compressor. The cross over valve cyclically supplied atmospheric air under pressure to a bottom or input end of one of the sieve beds while purging the other sieve bed by venting or drawing a vacuum at its input end. The first sieve bed was filled with a zeolite or other material which, in the case of an oxygen concentrator, adsorbed the nitrogen component of the atmospheric air allowing the oxygen to pass through an outlet at an upper or outlet end of the sieve bed. A small part of the separated oxygen was fed back to the outlet end of the purging sieve bed and the remainder was supplied downstream as the primary product gas.
The zeolite in the first bed was only able to adsorb a fixed amount of nitrogen, as determined by surface area, number of adsorption sites, and other characteristics of the zeolite material. If air continued to be supplied under pressure to the first molecular sieve bed after the zeolite material was saturated with adsorbed nitrogen, large amounts of nitrogen gas would be discharged through the outlet, i.e. a breakthrough is said to occur. To prevent contamination of the output oxygen gas by breakthrough nitrogen, the cycle time for the cross over valve was selected to reverse the pressurized and purging sieve beds before breakthrough occurred.
At the instant when the cross over valve stopped supplying air under pressure to the first sieve bed, the first sieve bed contained substantially pure oxygen near its outlet end. The oxygen concentration decreased along the bed from substantially pure oxygen to atmospheric air at the inlet end. The exact amount of substantially pure oxygen and the oxygen concentration drop off rate was determined by how close to breakthrough the cross over valve changed states.
To conserve this already separated oxygen, others have provided a valve to interconnect the outlet ends of the two sieve beds to transfer this left over substantially pure oxygen to the recently purged or evacuated bed. Although moving the substantially pure oxygen was advantageous, the pressure equalization valve was commonly held open after breakthrough. As the pressure in the pressurized sieve bed decreased, adsorbed nitrogen was released. This release of nitrogen caused the gas that was transferred by the equalization valve to have a nitrogen concentration which exceeded that of atmospheric air. Worse yet, the substantially pure oxygen was pushed toward the inlet end of the recently purged bed while the nitrogen rich gas remained adjacent the outlet end. The nitrogen contamination of outlet end zeolite increased the chances that it would eventually be sent downstream with the output product gas.
Others have placed a pressure equalization valve between the inlet ends of the two beds. However, this placement of the equalization valve again introduced the leftover pressurized gas into the recently purged bed in the wrong order. Atmospheric air was introduced first followed by oxygen rich gas. However, because the reduction in pressure caused nitrogen gas to be released by the molecular sieve, increasing amounts of nitrogen became intermixed with the previously separated oxygen reducing its purity. However, the molecular sieve material in the recently purged bed adsorbed the nitrogen component of the first introduced gas such that the nitrogen was held closer to the bottom or inlet end of the recently purged tank. Although the energy necessary to compress the gas that remained in the first bed was conserved with the bottom end pressure equalization valve, the separated air was again introduced in wrong order, i.e. lowest oxygen concentration first, highest oxygen concentration second. Because the substantially pure oxygen gas was transferred last, it became contaminated with desorbed nitrogen and much of it remained in the first bed when equilibrium was reached.
The present invention contemplates a new and improved PSA gas concentrator and concentrating method which overcomes the above referenced problems and others.